A structure known as a box structure represented schematically in FIG. 1A is a structural assembly, that is to say an assembly adapted to transmit mechanical loads, comprising two skins (101, 102) separated by stiffeners extending in two concurrent directions, wherein said longer stiffeners are generally called spars (111, 112) and shorter stiffeners are generally called ribs (121, 122).
In FIG. 1B, a straight section of these stiffeners shows an overall shape comprising two flanges (116, 118) separated by a core (115), wherein the skins (101, 102) are joined to the flanges (116,118) on the application surfaces (130) of said flanges. Said stiffeners make the structure more rigid, while the skins drain the mechanical loads to all said stiffeners. Thus, the quality of assembly of the skins with the stiffeners is of crucial importance for the mechanical transfer to occur. The stiffeners and the skins demarcate closed spaces, which are accessible by hatches (140) made in the stiffeners.
According to the prior art, particularly in the area of aeronautics, the skins are assembled to the stiffeners with rivet type fasteners. This technical solution does however have drawbacks. The use of rivets entails drilling holes into the flanges of the stiffeners, which weakens them. These holes are potential sources for the initiation of fatigue cracks, which is why said rivets are generally fitted with high interference in order to apply pressure on the walls of the holes made for them. The flow of forces transmitted between the skins and stiffeners use the application of caulking stress on said walls of holes. Thus, the use of materials that are unable to undergo plastic deformation such as composite materials with fiber reinforcement is not favorable, from the point of view of both the caulking strength and the possibility of high-interference fitting. Even in the presence of metal materials, the implementation of this assembly solution is lengthy and requires, at least for the assembly of one of the skins, the use of blind fasteners that can be installed by accessing only one side of said skin, and fasteners of this type are further expensive and difficult to install. Glued assembly solutions have been envisaged in the prior art.
These glued solutions are called interposition solutions. They consist in applying adhesive to the assembly interface between the skin and the flange, either both or one of the surfaces depending on the embodiment, then putting the two surfaces in contact under pressure, where said pressure is sufficient to make the adhesive flow so that it fills the interface between the two parts with even and definite thickness. However, while such solutions can be envisaged for assembling stiffened panels, that is to say stiffeners assembled with a single skin, these techniques are difficult or even impossible to apply on box structures. That is because if, with a stiffened panel, stiffeners can be glued one after the other to said skin, the assembly of the second skin of a box structure implies that all the stiffeners must be glued at the same time. Such an operation requires a large quantity of adhesive that must be applied and made to flow at the contacts, which cannot be envisaged when the surfaces concerned are large in view of the short open time for applying the adhesive and the very high and even pressure that must be applied to obtain sufficient flow of said adhesive and the absence of gluing defects.
The document U.S. Pat. No. 5,273,606 describes the assembly by gluing of a skin and a stiffened panel to that skin. The document considers that such a stiffened panel may be assimilated with a stiffener and does not describe the assembly by gluing of the flange of said stiffener on the skin. That is because the assembly method described in this document is applied at the periphery of the stiffened panel, by means of a hem made by folding an extra length of the skin over the periphery of said skin. Said hem presses against the periphery of the stiffened panel, and gluing is applied between the two sides opposite the hem. Thus, the assembly principle is only applicable between two panels, as gluing limited to the periphery would not allow the transfer of loads to the stiffeners.
The document JP 57 152917 describes a method of gluing a stiffener to a skin. However, the document does not describe the gluing of the flange of said stiffener to the skin.
These two documents of the prior art are aimed at solving the technical problem relating to gluing and consisting in obtaining a continuous film of adhesive with even thickness over the entire assembly interface. The solutions to that technical problem that have been proposed by these documents of the prior art consist in making spaces with a substantially calibrated thickness into which the adhesive is injected. These two documents of the prior art disclose the creation of these spaces directly in the assembled elements, by creating a hem or a boss that creates a space in the wing of the stiffener. Both the boss and the hem add to the complexity and cost of making the parts and add mass to the panel without making it stronger, which is particularly unfavorable in the aeronautics context.